Electroplating involves deposition of a metal layer onto a solid surface by means of electrolysis. It is carried out in a bath which may comprise fused salts or solutions of various kinds. In commercial practice aqueous solutions are typically used.
Most electroplating, e.g., most copper plating, is done by methods involving consumable electrodes. See, for instance, Modern Electroplating, A. A. Lowenheim, editor, The Electrochemical Society, Inc., Princeton, N.J., pp. 165-223, Third Edition. In consumable electrode plating processes, metal ions, derived typically from a salt, e.g., CuSO.sub.4, are caused to be deposited on a cathode in contact with the plating solution, and the corresponding number of ions enters the bath through dissolution of a metal, e.g., copper, anode that is also in contact with the bath. The overall process thus comprises a transfer of metal from the anode to the cathode.
Some metals, however, are typically plated by methods involving inert or insoluble anodes, i.e., anodes that are essentially not consumed in the plating process, and that thus are not substantial sources of cations for the plating process. Among these metals are chromium and gold. In these processes the concentration of metal ions in the plating bath is maintained within the appropriate range by addition to the bath of compounds that yield the appropriate metal ion upon dissolution of the compound in the bath. For instance, in chromium plating, the chromium is introduced into the bath in the form of CrO.sub.3, which is readily soluble in water. Similarly, gold plating is frequently carried out with inert anodes in cyanide solution, with the gold typically added in the form of AuCN or KAu(CN).sub.2. See, for instance, Y. P. Okinaka et al, U.S. Patent Application, Ser. No. 104,181, filed Dec. 17, 1979 for "Method of Replenishing Gold in Plating Baths."
The substitution of an inert (insoluble) anode plating process for a process using consumable electrodes often offers substantial advantages. For instance, in copper plating, e.g., in the manufacture of printed circuit boards, soluble copper anodes are currently used in systems operating typically at current densities of about 250-450A/m.sup.2, and result in typical plating times for circuit boards of 30-45 minutes. By carrying out the plating of circuit boards in a system employing insoluble anodes, product yield and uniformity of thickness of the deposit can typically be improved, and equipment maintenance be reduced. Furthermore, because of, e.g., decreased anode polarization, such a system potentially can employ higher current densities, in excess of 1000A/m.sup.2, resulting in plating times that are potentially significantly shorter than those typical of consumable anode systems.
Inert anode plating processes require replenishment of cations in the electrolyte. This is typically accomplished by addition to, and dissolution in, the electrolyte of cation-yielding compounds. However, some compounds that otherwise are very advantageous cation sources tend to form, in the electrolyte, aggregates that do not readily dissolve, and for this reason usually are not considered to be useful cation sources in inert anode plating. Improvements in inert anode plating processes that permit the use of such relatively poorly soluble compounds as cation sources are thus of considerable interest.